Valve Seat Recession

[malcolm]

I should know better. Though I've done it again and lost compression on the front cylinder on my GTL following a 900km drive flat out at 130km/h. Pulled the head off this afternoon so some photos for amusement. First the bad exhaust valve:



Then a good one that projects much further out of the cylinder head:



And the valve removed:



ANd finally the valve seat:



It's surprising how good the valve and valve seat still are. I wish I'd just adjusted the valve clearances rather than pulling the head off.

One thing that's struck me - if cylinder temperatures are higher with unleaded petrol, should we be running slightly bigger valve clearances?

The other thing that struck me is the overly lean standard mixture doesn't appear to be the problem - I was running a R5 carb that is only managing about 35 miles to the gallon (the R4 carb would be 40).

 

[mr-reno-139]

hi malcolm ,wasnt steve hennessy going on about valve clearances ?
found some brand new valves today thinking of you but forgot part numbers

all old school cars are sufferring this problem with modern petrol
worse with shim type OHC as checking and adjusting much trickier
remember the percentage of affected cars in regular use is shrinking
me, most of my every day stuff is diesel (which seems ok on veg oil !)

 

[malcolm]

I can't find anything from Steve, though looking up the MGB book the valve clearances are 0.4mm cold compared to 0.2mm for the R4. Though that too would have been written in the days before unleaded petrol.

Oddly enough I adjusted them recently - they were all very rattly prior to adjustment.

 

[Steve Hennessey]

Mr Reno, I'm always wittering on about something :roll: - the last time I mentioned valve clearances was to paul and pat in France.

Malcolm, I've seen valves and seats in a worse condition so not quite sure why you lost all compression on number 1. maybe to only way to solve this issue is to get unleaded exhaust seats fitted. I invested in this when I first got my 4 and have never had any problems - and I used to thrash it!!
The colour of the valves look fine so maybe your mpg went down as you have been using all that extra power:wink:. I must admit that I never did a mpg check between the 28IF and the 32IF carbs so can't comment - I was just grateful of the extra ooomph.

Now the head is off, will you consider having unleaded seats fitted?

Being an Imperialist, I set the tappets at 6 thou" inlet and 8 exhaust.

 

[John Doe]

Luckily leaded petrol (98) is still available over here. You guys are always talking about burned valves, compression lost etc. but I never came across this problem, R4's are continuously (with exception when we owned vw beetle for 4 years) in our family since '77 or '78 and never had any serious engine problems. Were we just lucky? Three years ago I had to drive very fast for 1100km (probably 80% on highways), this summer I did the same and the speeds were around 130km/h or higher all the time, no problems at all.

 

[malcolm]

We didn't have the problem before unleaded petrol - the lead lubricates the valve seats, and leaded petrol runs at a lower temperature. I gather that ceramic scum that builds up on the valves is quite abrasive, though it's the exhaust valves at either end of the engine that seem to fail - especially the front one. I guess the temperature makes them softer? I'll use an additive next time I do a long trip.

I guess the loss of compression was due to more than 0.20mm valve seat recession during the trip. Ought really to fit hardened valve seats, though I'm using the car to drive to work so just replaced the exhaust valves at each end and ground in the valves.

Removed some of the sharp edges while the head was off - exhaust ports were nasty. Hopefully I'll have more performance. At least she idles well now theres some compression.

 

[angel]

Valve clearances come into play at the warm up period, that is when engine parts are trying to reach their operational temperatures. That's why manufacturers give a gap with engine hot, otherwise the specifications would be, gap when cold, zero gap when hot. It is at the warm up period that this gap becomes minimum (or even zero). I doubt that we should increase exhaust valve clearances, it is an aluminum head and cools well. In contrast, on 2CVs running unleaded, all people had bad experience with the standard setting (0,25mm) and it is better to set them to 0,30mm.

The MGB has cast iron head, among others, that may be a reason why its valve clearance is greater. Incidentally, I never had any problems on an MGB on which I fitted hardened seats and left the clearances as per the manual.

You will be surprised, but modern "leaded" petrol does not actually contain tetra-ethylic lead. To confirm with later EU regulations, they have another additive (calcium based, if I remember correctly), hence the name LRP (Lead Replacement Petrol). Many people say it does not protect valve seats as well as lead, I didn't have any bad experience from this yet.

Some years ago i was discussing this issue with my friend Ingo Heitel, who told me that in Germany there were R4s having covered 150.000 kms+ on unleaded without VSR (we were talking about 845cc). I guess that the seats fitted at the factory were of good quality steel, so these engines are compatible with unleaded. My experience has proven it: about 80.000 kms on unleaded (on a head skimmed by 0,4mm and rather high coolant temperatures) and after checking valve clearances every 10.000 kms, all found normal (very slight increase).

 

[malcolm]

Once again that's brilliant Angel - I'd been wondering why hot clearances were often higher than cold clearances.

I wonder if the 1108cc engines are more succeptable to the problem than 850cc engines. I've never had a problem with an 850 either, even when driving hundreds of kilometers in a day, but it's the second time it's happened on an 1100 - both times following high speed driving across France.

Though no problems at all during normal use in the UK including motorway driving, just those really long high speed trips.

 

[angel]

I have always thought that 1108 cc engines did not actually have the build and material quality of 3-bearing units...what you say, Malcolm confirms this. Maybe Renault reckoned high quality valve seats were not necessary?
The only problems I have heard with 1108 engines (again from Ingo Heitel) was pinking when driving flat-out. It seems logical with 9,5:1 compression ratio.

 

[bantamallan]

A word on additives, I treat my GTL cruelly, only looking at it when something goes wrong or breaks. For the 6 months I used and additive, Castrol I think, but the tickover became very lumpy with a permanent missfire. I pulled the plugs out and found all of them fouled. Anew set of plugs and it was as good as new, or as good as 150k miles anyway. A sesion on a Crypton showed everything else , mixture, timing good. Since then no additive, no trouble
Allan